So can the OS.  A fiber is, at it's simplest, a thread that the OS can't
schedule.  As such, it must have a parent thread that schedules it.  Windows
allows multiple fibers to have the same parent thread.  Thus, a .Net mapping
of a thread still maps to a single OS thread.  A .Net mapping of a fiber
maps to a single OS fiber.

From MSDN:

"A fiber is a unit of execution that must be manually scheduled by the
application. Fibers run in the context of the threads that schedule them.
Each thread can schedule multiple fibers. In general, fibers do not provide
advantages over a well-designed multithreaded application. However, using
fibers can make it easier to port applications that were designed to
schedule their own threads."

=======
Ada does it's own thread scheduling, which is partly why the US DoD wanted
(note the past tense) to use Ada for weapons systems.

For what it's worth, there is a warning in the Beta 2 MSDN to not treat
pooled threads as OS threads.  From reading this warning, it appears that
the managed thread pool actualy is a pool of OS fibers.
<http://msdn2.microsoft.com/library/ms182291(en-us,vs.80).aspx >

Mike.